Objectives: The objective of this study was to investigate the relationship between the simultaneous 75 g Oral glucose tolerance test (OGTT), gestational diabetes mellitus (GDM), and fetal pancreatic circumference at 24–28 weeks of gestation. Methods: This prospective case–control study was conducted between September 2024 and February 2025 at our perinatology clinic, which provides tertiary health care services. The correlation between the 75 g OGTT, GDM, and pancreatic circumference was assessed by comparing fetal pancreatic circumference between the groups with and without GDM at the time of diagnosis. Results: A total of 130 pregnant patients were recruited for this` study, with 64 patients forming the GDM group and 66 patients forming the control group. Fetal pancreas circumference (7.0 cm vs. 6.4 cm, p < 0.001), fetal pancreas circumference percentile (88.5 vs. 52, p < 0.001), and the rate of fetal pancreas size >90th percentile (15.6% vs. 3%, p < 0.001) were significantly higher in the GDM group compared to the control group. Conclusions: Although our findings demonstrate a statistically significant correlation between fetal pancreatic circumference and GDM, diagnostic performance remains modest. Therefore, fetal pancreatic circumference should be interpreted as a supportive marker, such as family history, rather than a definitive marker for identifying individuals at risk for GDM.